1. Field of the Invention
The present invention relates to a demodulating device, a broadcasting system and a semiconductor device, and more particularly, to a demodulating device for demodulating a modulated signal, a broadcasting system for performing digital satellite broadcast communications, and a semiconductor device in which a circuit element for demodulating a modulated signal is integrated on an identical substrate.
2. Description of the Related Art
PSK (Phase Shift Keying) is known as one of digital modulation/demodulation techniques. PSK is a modulation technique in which phase is shifted as a parameter of carrier (carrier wave), and is widely used in the fields of satellite communications etc.
FIG. 16 shows a schematic configuration of a conventional PSK demodulator. The PSK demodulator 400 comprises a local oscillator 401, multipliers 402a and 402b, a π/2 phase shifter 403, a recovery section 404, and a phase noise correction section 405.
The local oscillator 401 generates a sinusoidal wave having the same frequency and phase as the carrier used on the transmitting side for modulation. The π/2 phase shifter 403 shifts, by π/2, the phase of the local oscillation signal generated by the local oscillator 401. The multiplier 402a obtains a product of the input signal and the local oscillation signal from the local oscillator 401. The multiplier 402b obtains a product of the input signal and the output of the π/2 phase shifter 403.
The recovery section 404 passes low-frequency components of the output signals from the multipliers 402a and 402b, then subjects the low-frequency components to A/D conversion to obtain digital signals corresponding to respective phase axes, and recovers the timing and the carrier, thereby demodulating the baseband signal.
Provided that the frequency component of the local oscillation signal generated by the local oscillator 401 is fL, the frequency component of the signal actually generated by the local oscillator 401 is fL±Δf, where Δf represents an unstable frequency part including frequency deviation, phase noise, etc.
Thus, the output of the local oscillator 401 includes an unstable frequency part (the oscillation frequency varies). Since the carrier is recovered using such a local oscillation signal, the baseband signal inevitably includes phase noise (phase jitter). If the phase noise is large, then the phase difference between I- and Q-channel signals correspondingly increases.
Accordingly, in cases where the phase noise is large, it is necessary that control be performed quickly following the phase noise in such a direction as to suppress the influence of the phase noise, so that a signal free from phase difference may be generated without delay. It is therefore essential that ordinary carrier recovery control be additionally provided with phase noise correction control, to bring symbols of the baseband signal to their normal symbol positions as quickly as possible.
As conventional phase noise correction control used in the illustrated PSK demodulator 400, first, a phase noise correction section 405 calculates a ratio of symbol variation in the phase direction to that in the amplitude direction, and generates a correction value based on the result of calculation. Then, based on the correction value thus generated by the phase noise correction section 405, the recovery section 404 rotates the phase of symbols, thereby suppressing the influence of the phase noise.
In the above conventional PSK demodulator 400, however, since the phase noise correction control is performed based on the ratio of symbol variation in the phase direction to that in the amplitude direction, the correction control is susceptible especially to noise that enters the radio signal via antenna, thus giving rise to a problem that the accuracy in the phase noise correction is low.
Further, in the conventional phase noise correction control, while a C/N (Carrier-to-Noise) ratio is low, a situation can arise where there is no spacing between adjacent symbols in the constellation showing the distribution of symbols. If this occurs, phase noise cannot be corrected at all, causing a lowering of the quality of radio communication control.